For the throngs of commuters using London's busy Waterloo Station every day, passing a large yellow sign reading "Bollocks to Brexit" has become part of the daily commute.

Flamboyant businessman Charlie Mullins, the chief executive of Pimlico Plumbers, put the sign up in protest against what he thinks will be a "disaster" for the British economy.

He is now defying calls for the slogan to be taken down.

"I believe I'm standing up for businesses," the spiky-haired 64-year-old told AFP in an interview at his London office -- above which the sign is displayed alongside a Union Jack flag.

"We're not prepared to see our businesses go down or suffer".

Mullins is a self-made man who is well known for his outspoken views and his Rod Stewart-like rock star haircut.

He left school at 15 to become a plumber and started his own business in 1979 -- a company that now employs 450 people and has a turnover of £40 million (45 million euros, $52 million).

'Car crash' Brexit

Flush with success, he is now turning his attention to trying to stop Britain from leaving the European Union on March 29.

He used to be a donor to the Conservative Party but is now funding the Liberal Democrats, the only mainstream party that is campaigning for a second referendum to block Brexit.

Leaving the EU is "a car crash waiting to happen" and "the worst thing to happen to the UK since the Second World War", he said, citing an estimated cost of £500 million a week that was provided last month by the Centre for European Reform think tank.

"We're going to lose a hell of a lot of migrants that we need here, high-skilled or low-skilled, they basically stop coming here," said Mullins.

The business owner said he was particularly angered by Prime Minister Theresa May's leadership in negotiations with Brussels.

"Two years she's been negotiating and we've gone no further forward. If anything, we've gone backwards," he said.

Along with a growing number of politicians, business people and civil society figures, Mullins is supporting a second referendum to reverse the result of the 2016 Brexit vote.

"I'm convinced that if there was another vote, Remain would probably win by 60-40," he said, giving a number that is not supported by recent opinion polls and would represent a major reversal from the 52 percent vote for Leave in 2016.

Local authorities have received complaints about his sign but he has said he has no intention of taking it down.

"It's freedom of speech," he said, adding: "Until they put me in prison for it, it's not coming down."